ORDER POLYPI POLYPIFERI. 517 
THETHYA, Lam. 
The interior of which is altogether bristled with long sili- 
ceous spirals, which are united on a central nucleus equally 
siliceous. ‘Their crust presents, as in the sponges, two orders 
of holes ; the one closed by a sort of trellis-work, must be for 
the entrance of water; the other, gaping, destined for its 
exit. 
We also place at the sequel of the Alcyonia, 
SPONGIA, L. 
The sponges are marine fibrous bodies, which exhibit 
nothing perceptibly, but a sort of thin, fine, gelatine, which 
dries up, and scarcely leaves any trace, and in which, as yet, 
no polypi, or any other moveable part, have been observed. 
It has been said that the living sponges undergo a sort of 
tremor, or contraction, when they are touched ; that the pores 
with which their superficies is pierced, palpitate in some 
degree; but the existence of such movements has been con- 
tested by Mr. Grant, and other writers. 
The sponges assume innumerable forms, each according to 
its species, as those of shrubs, trumpets, vases, tubes, globes, 
fans. , 
Every one is acquainted with the common sponge, spongia 
officinalis, which is found in great brown masses, formed of 
very fine fibres, flexible, elastic, and pierced with a great 
number of pores, and small irregular conduits opening one 
into the other. 
